Layout style guide: Create separate section for markup HTML tags

[mao-sz]

Because

Currently we document that block-level HTML tags used for markup must be surrounded by a single blank line each side, but it's documented in the Note Boxes section. Since this style rule applies to all block-level markup HTML tags, not just note boxes, this should be its own section.

This PR

• Extracts style guide content on markup HTML tags to own main section

Issue

N/A

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[01zulfi]

lgtm